Recombinant African swine fever virus Envelope protein p54 (Ken-138)

What is the structural composition of the p54 protein in ASFV?

The p54 protein is a 25-kDa type I membrane-anchored polypeptide encoded by the E183L gene of ASFV. It contains a transmembrane domain near the N-terminus that anchors it to viral membranes. The protein forms disulfide-linked homodimers through its unique luminal cysteine residue . The full-length protein consists of 175 amino acids with the sequence: MDSEFFQPVYPRHYGECLSPTSTPSFFSTHMCTILVAIVVLIIIIIVLIYLFSSRKKKAA APAIEEEDIQFINPYQDQQWAGATPQPGTSKPAGATTGNVGKPITDRPATDRPVTNNPVT DRLIMATGGPAAASAPSAELYTTATTQNTASQTMPAVEALRQRSTYTHKDLENSL . This structural protein is located in the inner envelope of virions and plays critical roles in viral assembly.

How does the p54 protein contribute to ASFV morphogenesis?

The p54 protein is essential for ASFV morphogenesis, particularly during the earliest stages of viral assembly. Studies using a lethal conditional recombinant virus (vE183Li) have demonstrated that repression of p54 synthesis arrests virus morphogenesis at a very early stage, even prior to the formation of precursor membranes . The protein is critical for recruiting and transforming endoplasmic reticulum (ER) membranes into the precursors of the viral envelope. When p54 synthesis is repressed, virus factories appear as discrete electron-lucent areas essentially free of viral structures, while aberrant zipper-like structures formed by unprocessed core polyproteins accumulate outside the assembly sites . This indicates that p54 is fundamental to initiating proper viral assembly.

What epitopes have been identified on the p54 protein, and what is their significance?

Several important epitopes have been identified on the p54 protein:

• Linear B-cell epitope 76QQWVEV81: This recently identified epitope is highly conserved among genotype II ASFV strains. Through alanine-scanning mutagenesis, researchers determined that 76QQWV79 serves as the core binding site. This epitope can react with antibody-positive serum from naturally infected pigs, confirming it as a natural linear B-cell epitope .

• Transmembrane and cytoplasmic domains: The N-terminal region contains a transmembrane domain critical for membrane anchoring, while cytoplasmic domains interact with host cellular components.

The identification of these epitopes is significant for:

• Developing more specific diagnostic tools

• Designing targeted vaccines against conserved regions

• Understanding antibody-mediated immunity against ASFV

How does p54 interact with host cellular components during ASFV infection?

The p54 protein engages in multiple critical interactions with host cellular components:

• Endoplasmic reticulum interaction: p54 behaves as a type I membrane-anchored protein that targets ER membranes when expressed in transfected cells. This interaction is crucial for viral morphogenesis as p54 recruits and transforms ER membranes into viral envelope precursors .

• Dynein interaction: Following viral internalization, p54 interacts with the host protein dynein, facilitating intracellular transport of the virus . This interaction suggests p54's involvement in viral trafficking mechanisms within infected cells.

• Apoptosis regulation: p54 activates caspase-3 during the early stages of ASFV infection, contributing to cellular apoptosis, which represents the first indication that ASFV induces cell death .

• Virion attachment: Together with p30, p54 mediates binding of the virion to target cells , initiating the infection process.

These interactions collectively demonstrate p54's multifunctional role in ASFV infection beyond just structural components.

What are the current experimental approaches to evaluate p54's role in ASFV pathogenesis?

Researchers employ several sophisticated experimental approaches to understand p54's role in ASFV pathogenesis:

• Conditional lethal mutants: Development of viruses like vE183Li with inducible p54 expression allows researchers to study the effect of p54 repression on viral morphogenesis in controlled conditions .

• Electron microscopy analysis: Ultrastructural examination of virus factories in the presence and absence of p54 reveals its role in membrane recruitment and transformation .

• Protein-protein interaction studies: Techniques such as co-immunoprecipitation and proximity ligation assays help identify host factors that interact with p54 during infection.

• Cryo-electron microscopy: Advanced structural studies reveal the position and conformation of p54 within the viral particle.

• CRISPR/Cas9 technology: This approach has been used to engineer recombinant viruses expressing modified versions of p54 to study its function. For example, researchers have created pseudorabies recombinant viruses expressing ASFV CD2v and p54 proteins (PRV-∆TK-(CD2v)-∆gE-(p54)) to evaluate immunogenicity .

How does genetic diversity in p54 sequences impact viral phylogeny and vaccine development?

The genetic diversity of p54 has significant implications for both viral classification and vaccine development:

• Phylogenetic utility: The E183L gene encoding p54 has been successfully used to classify ASFV isolates into different genotypes and subclusters. This high-resolution differentiation can be achieved because p54 contains sufficient genetic variability to distinguish between viral strains .

• Conserved epitopes: Despite variation, certain regions like the 76QQWVEV81 epitope remain highly conserved, particularly among genotype II strains . These conserved regions represent potential targets for broadly protective vaccines.

• Geographical clustering: Analysis of p54 sequences helps track the geographical spread of ASFV. For example, studies have used p54 gene analysis to characterize isolates from South Sumatra and Lampung, confirming they belong to genotype II .

• Vaccine design challenges: The diversity in p54 sequences necessitates careful consideration when designing subunit vaccines. Researchers must select conserved epitopes or include multiple variants to ensure broad protection against diverse ASFV strains.

Table 1: Genotypic Classification Using p54 Gene Analysis

How can researchers optimize p54-based ELISA for ASFV diagnosis?

Optimization of p54-based ELISA for ASFV diagnosis requires careful consideration of several parameters:

• Antigen preparation:

  • Express recombinant p54 in E. coli as a fusion protein with appropriate tags (MBP or His)

  • Purify using affinity chromatography (e.g., MBP Trap column)

  • Use optimal coating concentration (4 μg/mL in phosphate-buffered saline, pH 9.6)

• Assay format selection:

  • Competitive ELISA (cELISA) using p54-specific monoclonal antibodies shows superior performance

  • In a validated cELISA, a threshold of 20% inhibition achieved 92.5% sensitivity and 98.9% specificity

• Antibody development:

  • Generate monoclonal antibodies against p54 by immunizing BALB/c mice with purified protein

  • Screen hybridomas by indirect ELISA and confirm specificity via Western blot and immunofluorescence

  • Select antibodies targeting conserved epitopes for broader detection capabilities

• Validation using reference panels:

  • Test with well-characterized positive and negative samples

  • Include samples from different geographical regions and ASFV genotypes

  • Conduct ROC analysis to determine optimal cut-off values

• Cross-reactivity assessment:

  • Verify no cross-reactions with other swine pathogens (CSFV, PRRSV, FMDV, PEDV)

  • Ensure specificity across different sample types (serum, blood, tissue)

Research shows that p54-based ELISAs can achieve an area under the ROC curve of 0.982, indicating excellent diagnostic capability .

What are the methodological considerations when developing a real-time PCR assay targeting the p54 gene?

Developing an effective real-time PCR assay targeting the p54 gene requires several methodological considerations:

• Primer and probe design:

  • Target highly conserved regions of the E183L gene to ensure detection across multiple ASFV genotypes

  • Design primers that avoid regions with known polymorphisms

  • Optimize primer and probe concentrations through titration experiments

  • The VNUA-p54 assay represents a successful implementation of this approach

• Sensitivity optimization:

  • Determine the limit of detection (LOD) through serial dilutions of reference material

  • The VNUA-p54 assay achieved an LOD of 2.63 genome copies, slightly better than the compared p72-based assay (3.29 copies)

  • Optimize cycling conditions to maximize sensitivity while maintaining specificity

• Specificity testing:

  • Test against other swine pathogens to ensure no cross-reactivity (CSFV, FMDV, PRRSV, PEDV)

  • Validate across multiple ASFV strains representing different genotypes

  • The VNUA-p54 real-time PCR detected fifteen different ASFV reference strains across genotypes I, II, and V

• Sample type validation:

  • Verify performance across different sample types (blood, serum, tissues)

  • Optimize extraction methods for each sample type

  • Studies show the p54 PCR assay works well with whole blood, serum, spleen, and kidney samples

• Internal controls:

  • Include appropriate internal controls to monitor for PCR inhibition

  • Consider duplex assays that simultaneously detect host genetic material

Table 2: Performance Comparison of p54-Based Real-Time PCR with Other Assays

What experimental designs are most appropriate for evaluating p54-based vaccine candidates?

Evaluating p54-based vaccine candidates requires robust experimental designs that can properly assess safety, immunogenicity, and protective efficacy:

• Randomized controlled trial design:

  • Randomly assign test subjects to experimental and control groups to minimize bias

  • Include appropriate control groups: negative control (PBS), vector-only control, and positive control (if available)

  • Consider both between-subject and within-subject designs depending on the experimental question3

• Dose-response studies:

  • Test multiple antigen doses to determine optimal immunization regimen

  • Assess both humoral and cellular immune responses at each dose level

  • Determine minimum protective dose through challenge studies

• Prime-boost strategies:

  • Compare different vaccination schedules (e.g., homologous vs. heterologous boosting)

  • Evaluate the combination of p54 with other ASFV antigens such as p30, p72, or CD2v

  • Goatley et al. found that using rAd prime and MVA boost as a delivery system with multiple antigens including p54 protected pigs from fatal disease

• Animal model selection:

  • Initial screening in mice to assess basic immunogenicity and safety

  • Progression to pig models for efficacy evaluation

  • C57BL/6 male mice have been successfully used for initial immunogenicity testing

• Challenge model standardization:

  • Use well-characterized ASFV challenge strains

  • Standardize challenge dose, route, and timing post-vaccination

  • Monitor clinical signs, viremia, viral load in tissues, and survival rates

• Longitudinal monitoring:

  • Collect samples at multiple time points to assess durability of immune responses

  • Monitor antibody titers, neutralizing activity, and T-cell responses over time

  • Assess long-term protection through delayed challenge studies

How should researchers approach the combination of p54 with other ASFV antigens in vaccine formulations?

Combining p54 with other ASFV antigens requires strategic approaches to maximize protective efficacy:

• Rational antigen selection:

  • Combine p54 with antigens that induce complementary immune responses

  • Key combinations include p54 with p30, p72, and CD2v

  • Research indicates that p54 alone often provides insufficient protection, but combinations can be more effective

• Antigen presentation strategies:

  • Express multiple antigens as fusion proteins or as separate entities

  • Consider co-expression in viral vectors or co-administration of multiple subunit proteins

  • PRV-∆TK-(CD2v)-∆gE-(p54) represents a successful example of co-expression of CD2v and p54 in a viral vector

• Delivery system optimization:

  • Evaluate different delivery platforms (viral vectors, DNA vaccines, protein subunits)

  • DNA-protein vaccination combinations may enhance both cellular and humoral immunity

  • Fusion of p54 with ubiquitin enhanced cellular immune responses by modifying antigen presentation

• Epitope-focused approach:

  • Target conserved epitopes across multiple antigens

  • Combine specific epitopes rather than full-length proteins where appropriate

  • The identified B-cell epitope 76QQWVEV81 on p54 could be combined with key epitopes from other proteins

• Adjuvant compatibility:

  • Select adjuvants that enhance responses to all antigens in the combination

  • Test different adjuvant formulations to optimize immune responses to the antigen combination

Experimental data has shown that combining eight different antigens including p54 (B602L, B646L/p72, CP204L/p30, E183L/p54, E199L, EP153R, F317L, and MGF505-5R) using rAd prime and MVA boost can protect pigs from fatal disease after challenge with virulent ASFV .

What methodological approaches can improve the expression and purification of recombinant p54 for vaccine development?

Optimizing the expression and purification of recombinant p54 requires specific methodological approaches:

• Expression system selection:

  • Bacterial systems: E. coli ER2523 strain has been successfully used with MBP-tagged p54

  • Mammalian systems: HEK-293T cells provide proper folding and post-translational modifications

  • Insect cells: Baculovirus expression systems offer high yield with eukaryotic processing

• Construct design optimization:

  • Remove the transmembrane domain to improve solubility

  • Use fusion tags that enhance solubility and facilitate purification (MBP, His)

  • Consider codon optimization for the expression system

• Purification strategy development:

  • Implement multi-step purification protocols:

    • Affinity chromatography using pre-packed MBP Trap columns for MBP-tagged p54

    • Size exclusion chromatography to remove aggregates

    • Ion exchange chromatography for final polishing

• Protein quality assessment:

  • Verify identity and integrity by Western blot with p54-specific antibodies

  • Assess purity by SDS-PAGE and mass spectrometry

  • Confirm proper folding through circular dichroism or thermal shift assays

• Stability enhancement:

  • Optimize buffer conditions to prevent aggregation and degradation

  • Consider lyophilization for long-term storage

  • Add stabilizing agents such as glycerol (50%) for frozen storage

Protocol example: For MBP-tagged p54 expression:

• Insert P54/E183L gene (excluding the transmembrane domain) into pMAL-c5x vector using NdeI and EcoRI restriction sites

• Transform into DH5α E. coli competent cells

• Induce expression with IPTG to obtain an approximately 65kDa fusion protein

• Purify using affinity chromatography

• Confirm identity by Western blot and sequence verification

How should researchers analyze the correlation between p54 antibody responses and protection against ASFV challenge?

Analyzing the correlation between p54 antibody responses and protection requires sophisticated statistical and immunological approaches:

• Comprehensive antibody profiling:

  • Measure multiple antibody parameters:

    • Total p54-specific antibody titers by ELISA

    • Neutralizing antibody titers using in vitro neutralization assays

    • Antibody avidity indices to assess maturation of response

    • Epitope-specific antibodies targeting identified regions like 76QQWVEV81

  • Determine antibody isotypes and IgG subclasses to characterize the response quality

• Survival analysis techniques:

  • Apply Kaplan-Meier survival analysis with log-rank tests to compare protection between groups

  • Use Cox proportional hazards models to identify antibody parameters associated with survival

  • Integrate virological outcomes (viral load reduction) with survival data

• Multivariate statistical approaches:

  • Employ principal component analysis to reduce dimensionality of antibody response data

  • Use discriminant analysis to identify antibody parameters that best distinguish protected from unprotected animals

  • Apply receiver operating characteristic (ROC) analysis to determine predictive antibody thresholds

• Mixed-effects modeling:

  • Account for individual variation and repeated measures in longitudinal studies

  • Model the relationship between antibody kinetics and protection outcomes

  • Incorporate both fixed effects (vaccination, challenge dose) and random effects (individual animal)

• Immune correlates identification:

  • Calculate correlation coefficients between antibody metrics and virological outcomes

  • Establish threshold antibody levels associated with protection

  • Validate identified correlates across multiple experiments and ASFV strains

Example research approach from the literature:
When evaluating PRV-∆TK-(CD2v)-∆gE-(p54), researchers detected robust serum IgG responses against both CD2v and p54 on day 7 after intramuscular vaccination, with continued monitoring through day 14 to establish response kinetics .

What experimental design considerations are crucial when comparing different p54 protein variants for immunogenicity?

When comparing different p54 protein variants for immunogenicity, several experimental design considerations are crucial:

• Variant selection strategy:

  • Include variants representing different ASFV genotypes or strains

  • Consider naturally occurring variants vs. rationally designed modifications

  • Include variants with mutations in known epitopes like 76QQWVEV81

• Controlled production methods:

  • Express all variants using identical systems and purification protocols

  • Verify comparable purity, conformation, and concentration

  • Characterize each variant using consistent analytical methods

• Study design selection:

  • Consider repeated measures (within-subject) designs when possible to reduce individual variability

  • Use randomized block designs to control for confounding variables

  • Power analysis to determine appropriate sample sizes based on expected effect sizes3

• Comprehensive immune assessment:

  • Measure both B-cell responses (antibody titers, neutralization capacity)

  • Assess T-cell responses (proliferation, cytokine production)

  • Evaluate innate immune activation (cytokine profiles, dendritic cell activation)

• Advanced statistical analyses:

  • Apply mixed ANOVA models for repeated measures designs

  • Use Bonferroni or Tukey corrections for multiple comparisons

  • Consider non-parametric alternatives when assumptions for parametric tests are violated3

• Controls and standardization:

  • Include positive controls (known immunogenic proteins) and negative controls

  • Standardize immunization protocols (dose, route, schedule)

  • Use consistent adjuvants across all variants unless adjuvant is a variable of interest

• Cross-reactivity assessment:

  • Test antibodies raised against each variant for cross-reactivity with other variants

  • Evaluate potential for cross-protection against different ASFV strains